Post navigation

Technical Report

Introduction

Reflex Photonics’s LightABLE SR4 parallel optical engine enables high-speed optical communication on multi-mode fibres at data rates up to 10.3125 Gb/s per channel. The module has 4 optical transmit and 4 optical receiver channels to support full duplex operation at a total bandwidth over 40 Gb/s.
A sample device has been recently evaluated by CERN, using Reflex Photonics’ LightABLE SR4 Demo Box (LHD040018312101) [now called Optical Tester SR4 40G-10G] which provides access to one TX and RX channel over 4 high-speed electrical connections through SMA connectors, as well as low speed control signals (I2C) for module configuration (figure 1). Transmitter optical and receiver electrical eye diagrams have been taken at 5 and 10 Gb/s and bit error rate (BER) tests have been carried out in various configurations to assess the performance of the LightABLE optical engine.

TheLightABLE™ LM SR4 with integrated microcontroller is a low profile optical transceiver that includes 4 transmit and 4 receive channels in a parallel fiber configuration.

2 Functional tests

2.1 Transmitter eye diagram

To test the dynamic performance of the transmitter, eye diagrams were measured using the setup shown in figure 2. The transmitter under test was fed by pseudo-random bit sequences (PRBS-7 and PRBS-31) which were produced by an Agilent J-BERT running at 5 or 10 Gb/s. The output of the transmitter was connected to a passive optical fanout allowing to connect the oscilloscope to transmitter channel 3. During the tests the module was operating with the power-on default settings. The eye diagrams captured at 5 and 10 Gb/s are shown in figure 3. The eyes are sufficiently open at both speeds and the impact of the pseudo random pattern length on the jitter is negligible. There is more deterministic jitter at 10 Gb/s (19 ps vs. 9 ps at 5 Gb/s), but the measured total jitter of 32 ps remains below the specified typical value. At 10 Gb/s the measured average optical power and extinction ratio were 1 dBm and 4.6 dB, respectively.

2.2 Receiver eye diagram

Receiver eye diagrams were captured using the test setup shown in figure 4. The optical signal was produced by a commercial SFP+ module (Avago, AFBR-709SMZ, S/N AA15163PCLF) plugged into a Module Compliance Board (MCB). The SFP+ module was fed by a pattern generator (Agilent J-BERT) running at 5 or 10 Gb/s. The optical signal coming from the SFP+ module was connected to receiver channel 3 using an optical fanout. The electrical output of the receiver module was connected to a high-speed oscilloscope. During the tests the module was operating with the power- on default settings. The optical eye diagrams at the output of the SFP+ module and the receiver’s electrical eye diagrams are shown in figure 5 and 6, respectively.

Looking at the eye diagrams captured at 5 Gb/s one can observe some ringing following both the rising and falling edges. The impact of this ringing is limited mostly to one unit interval and the resulting inter-symbol interference (ISI) remains low. At 10 Gb/s however, the ISI increases and the eye diagram closes as the test pattern gets longer (PRBS-31). This type of distortion may come from some discontinuities along the signal path of the receiver (e.g. PCB traces).

Figure 6: Electrical eye diagrams measured on channel 3 of the receiver module using pseudo random bit sequences (PRBS-7 and PRBS-31) running at 5 and 10 Gb/s. The measurements were carried out with the default receiver set- tings.

2.3 Bit error rate test

To complete the evaluation of the receiver of the Reflex Photonics LightABLE SR4 engine we carried out BER tests. The receiver sensitivity tests were carried out using the experimental setup shown in figure 7. The pattern generation (PG) and error detection (ED) are performed by an Agilent J-BERT. The LightABLE SR4 demo box was connected to the J-BERT using SMA cables. The optical signal of the reference SFP+ transmitter was going through a variable optical attenuator (VOA) and a fan-out cable assembly before reaching the receiver channel of the LightABLE engine. The average optical power of the attenuated signal was measured using the built-in power sensor of the VOA. The extinction ratio of the reference SFP+ transmitter was measured beforehand using a high-speed oscilloscope (see section 2.2). The results in figure 8 show that the tested device meets the vendor’s maximum receiver sensitivity specifications of -9 dBm at 10 Gb/s (option 1) with ample margin.

Finally, the test setup shown in figure 9 was used to measure the uplink (LightABLE SR4 transmitter to SFP+ receiver) performance. The setup is similar to the one described in the previous paragraph. The demo box is connected to the PG using SMA cables. The reference receiver is plugged into an SFP+ module compliance board (MCB) which in turn is connected to the ED inputs using SMA cables. The results obtained using transmitter channel 3 and the reference curves measured with the SFP in optical loop-back are shown in figure 10. The measured sensitivity values are within a few tenth of a dB confirming the negligible impact of the transmitter eye diagrams shown in section 2.1 on the system bit error rate.

Figure 9: Experimental setup used for measuring bit error rate of an optical link using the LightABLE SR4 engine as transmitter and a reference SFP+ module as receiver.

Figure 10: Bit error rate test results measured using the LightABLE SR4 engine as a transmitter and a reference SFP+ module as receiver.

3 Conclusions

Reflex Photonics’ LightABLE SR4 parallel optoelectronic engine supports multi-mode links with up to 4 full duplex channels operating at a maximum data rate of 10.3125 Gb/s per channel. A sample device has been tested in the laboratory in order to assess its performance. The measured optical eye diagrams show good transmitter performance independent from the length of the test pattern employed. The receiver performance seems to suffer from the quality of the electrical signal which gets worse as the data rate and/or the test pattern length increase. Nevertheless, the receiver sensitivity specification is met with sufficient margin.